“We have a hard time working on this body and notthinking about it as a planet.” — CHRISTOPHER RUSSELLVesta: More planet than asteroid

By Nadia Drake

The enormous asteroid Vesta is more like
a small, rocky planet than other space
rocks wandering around the asteroid belt
bet ween Mars and Jupiter. Among other
planetlike characteristics, Vesta’s interior is probably divided into layers like
Earth’s — and scientists have detected
traces of an ancient magnetic field.

“We have a hard time working on
this body and not thinking about it as
a planet,” said UCLA’s Christopher
Russell, principal investigator of the
Dawn spacecraft, which has been buzzing around Vesta since July.

Like Earth, Vesta probably has an
iron core, a mantle and a crust. Dawn
measurements suggest that the core’s
radius is bet ween 107 and 113 kilometers,
Carol Raymond of the Jet Propulsion
Laboratory said on March 22. Vesta is
about 530 kilometers across, so the core
occupies almost half its diameter. And
new gravity maps from Dawn reveal
areas in the crust where there’s “likely
mantle material close to the surface,”
Raymond reported.

Other reports relied on data from bits
of Vesta that had crashed to Earth. Two
chips off the old Vestal block contain
traces of a strong magnetic field, said
Roger Fu, an MI T graduate student who
studied meteorites known as Millbillil-lie (which landed in Australia in 1960)

This image of Vesta, based on datafrom the Dawn spacecraft, shows thetopography of the south polar regionwith the asteroid’s curvature removed.

and ALHA 81001 (found in Antarctica in
1981). The field’s strength suggests that
an active, liquid metallic core generated
the magnetic signature, which became
locked into Vesta’s crust as the asteroid
cooled. “There was a magnetic field on
the surface of Vesta after 3. 6 billion years
ago, and there probably still is,” Fu said.

Vesta also bears multiple scars from
two giant impacts, including mammoth
basins in its south pole and equatorial
troughs that formed as the rocky crust
reverberated from the impact.

The larger basin, called Rheasilvia, is
well-known. It is 505 kilometers across,
consuming essentially the entire south
pole, and hosts one of the tallest mountains in the solar system, which towers
20 kilometers above the basin’s floor.

But the second impact basin, beneath
and slightly to the side of Rheasilvia,
is a newly named structure called
Veneneia, Paul Schenk of the Lunar and
Planetary Institute in Houston reported.
It is about 395 kilometers across. The
double bull’s-eye at the south pole suggests that Vesta must be incredibly
resilient, Schenk said.

Even so, the impacts probably pene-trated Vesta’s crust and scattered miner-als that normally live deep undergroundover the surrounding surface. Scientistswill continue studying and characteriz-ing these details because even thoughVesta might resemble Earth, it embod-ies one crucial difference: Pages from itshistory aren’t erased. Instead, the aster-oid’s biography still contains records ofits evolution dating back to the dawn ofthe solar system.
the solar system.

Action on Saturn moon
Dione, a lesser-known Saturnian
moon, has been— and might still
be —geologically active. Evidence
from the Cassini spacecraft suggests that the small moon isn’t just
a staid icy rock, said Bonnie Buratti
of the Jet Propulsion Laboratory on
March 19. For one thing, charged
particles emanate from the moon’s
surface. Other areas are riven
with what Buratti calls “paleo-tiger
stripes”— features reminiscent of
the steamy fractures on another
moon of Saturn, Enceladus. There’s
a feature that looks suspiciously
like a cryovolcano, or something
that spews ice and other cold
materials instead of lava; nearby
areas are smooth, suggesting that
they might have been covered by a
frigid eruption. And there’s evidence
for a tenuous oxygen atmosphere.
Such observations suggest “recent
and/or ongoing activity on Dione,”
Buratti said. — Nadia Drake

Bright spots after Titan’s rain
Titan, a behemoth Saturnian moon
with its own weather systems,
has tossed scientists another
curveball. Though seasonal rains
wet and darken Titan’s surface,
some areas become brighter after
a storm, Jason Barnes of the University of Idaho reported on March
19. He and colleagues studied
Cassini images of Titan after a
September 2010 cloudburst. After
a dark phase that lasted for a few
weeks, two areas, Hetpet Regio
and Yalaing Terra, became whiter.
The light terrain persisted for
about a year before fading. Barnes
thinks that evaporating liquids
might cool Titan’s surface, causing
frost to form. — Nadia Drake